I am not an expert on this and I've never used BigTable or GCP before, but here are my two cents:
1) Did you test the read speed? Is it also very slow compared to writing?
2) Did you try using an HBase/Bigtable client (in the same GCP container as your JanusGraph instance) to write to your BigTable cluster? If it's also very slow then the problem might be with your network or other setups.